Sialic acids are a family of 9 carbon sugars (belonging to a larger family of nonoses, or nonulosonates) expressed in the tissues of every vertebrate and several “higher-order” invertebrates (1). Sialic acids serve a wide variety of biological roles, including modulating several aspects of immune function (2). For example, cell surface-associated sialic acid inhibits complement activation. As an example of immune regulation, sheep erythrocytes are resistant to lysis by the alternative pathway because surface sialic acids increase the affinity of factor H (fH; inhibitor of the alternative pathway) (3). Neuraminidase treatment of sheep erythrocytes then reduces the affinity of fH, which permits complement activation and promotes hemolysis. Recent work showed that fH C-terminal domains 19 and 20 bound simultaneously to C3b (complement factor that binds microbial cell surfaces) and glycosaminoglycans (including sialic acids), respectively, on host cells, which served to inhibit the alternative pathway (4). Loss of sialic acids decreased fH binding and enhanced activation of the alternative pathway. Typically, fH binds vertebrate cell surfaces via sialic acids to allow preferential protection of host cells (i.e. reduce complement-mediated damage).
Many microbes express sialic acids, as well as other unique microbial nonulosonates (i.e. legionaminic (Leg) and pseudaminic (Pse) acid), on their surfaces that contribute to pathogenesis in several ways including subversion of complement activation, promoting biofilm formation and facilitating colonization (5). Some pathogens such as Neisseria gonorrhoeae, Haemophilus influenzae, Histophilus somni (Haemophilus somnus) and group A N. meningitidis lack the ability to synthesize sialic or nonulosonic acids, but scavenge these molecules (such as Neu5Ac or Neu5Gc, or the CMP-activated form CMP-Neu5Ac) from the host. Other pathogens, for example, Escherichia coli K1, Streptococcus agalactiae, groups B, C, W, and Y N. meningitidis, Campylobacter jejuni and certain Leptospira, can synthesize nonulosonic acids such as Neu5Ac, Leg5Ac7Ac or Pse5Ac7Ac de novo. Sialylation of gonococcal lacto-N-neotetraose (LNT) lipooligosaccharide (LOS) enhances resistance of N. gonorrhoeae to complement-dependent killing by decreasing binding of IgG against select bacterial targets such as the porin B (PorB) protein (6), which attenuates the classical pathway. LNT LOS sialylation also enhances fH binding, which results in inhibition of the alternative pathway (7).
U.S. Pat. No. 6,096,529 and U.S. Pat. No. 6,210,933 disclose that LacNAc may be modified with a sialic acid analogue using a recombinant sialyltransferase derived from N. gonorrhoeae or N. meningitidis. These patents do not specifically mention that the sialic acid analogue could be Leg5Ac7Ac.
U.S. Pat. No. 6,168,934 discloses the proposition that it is possible to sialylate LacNAc with a sialic acid analogue using an appropriate sialyltransferase (see col. 24, lines 7-66). The patent does not specifically disclose the use of a sialyltransferase from Neisseria spp. The patent also does not specifically mention that the sialic acid analogue could be Leg5Ac7Ac.
N. gonorrhoeae has become resistant to almost every conventional antibiotic. Over the past 3 years, resistance to ceftriaxone has ushered in an era of potentially untreatable gonorrhea. There is an urgent need for novel therapeutics and vaccines against this disease. LOS sialylation is an important aspect of gonococcal pathogenesis and isogenic mutants that lack the ability to sialylate their LOS are at a disadvantage in vivo compared to their wild-type counterparts (31). Disabling the ability of gonococci to siaylate their LOS represents a novel prophylactic or treatment strategy.